Timestamps on multimedia data streams enable time information to be associated with the data representing audio, video or other time dependent media. Recorded streams may require alteration as to rate of play, synchronisation with other streams or editing. The use of timestamps embedded in the streams provides a marker for controlling such operations.
For example timestamps embedded in a data stream can be used to meter the playback rate to ensure that the stream runs at the desired rate. Additionally if several streams are recorded simultaneously and require editing, then operations such as cuts may be required at a particular recorded time on each stream. The use of a timestamp ensures that the cuts can be precisely synchronised.
Timestamps are used within current video tape formats and DAT formats. Timing information in actual time (GMT) is typically laid down with the audio and video streams as they are recorded. Subsequent editing of the streams results in a time track with discontinuities where cuts have been made, and `false` timing data where inserts have occurred, so that during subsequent playback the timestamps do not bear a simple relationship to the times at which the data should be played.
To avoid this a fresh time track may be laid down when editing, with the time at which the edits were made. The original timestamps are thus overwritten so that a new consecutive series of timestamps is available. This approach loses all information about when the original recordings were made, so that additional tracks cannot be synchronised with the edited sequence, further editing will lead to cumulative errors, and recordings cannot be indexed by when they were made.
Instead of timestamping with actual time (GMT), relative timestamps can be used. These are relative times with independent origins and are linked to every data segment (whether audio or video) by the hardware that captures the data. These timestamps often exhibit drift since the electronic clocks in different hardware do not necessarily run with sufficient accuracy. When streams are edited, new sets of relative timestamps are laid down so that the playout software does not have to take care of discontinuities where the edits have been made.